My Anytime BT connection is always very quick, they do disconnect you if your connection has been inactive for sometime (ie when you are reading a web page as oposed to clicking away). I haven't timed it but I'd guess this happens after 10min or so??
Other than that, if I am frequently clicking links I reckon I get around two hours before having to redial.
You can get software that pings the server (not sure about the terminology) so it thinks the connection is active. Unfortunately I don't know any names for this software but if you suspect this might be the cause of your frequent disconnections it might pay you to start
Googling or try
TUCOWS to find it.